1. Field of the Invention
This invention relates to a method of analyzing and optimizing design of integrated circuit (IC) designs. In particular, the present invention relates to a method of efficiently characterizing IC design modules using the Synopsys Design Compiler.
2. Description of the Related Art
Today, the design of most digital integrated circuits (IC""s) is a highly structured process based on an HDL (Hardware Description Language) methodology. FIG. 1 illustrates a simplified flowchart representation of an IC design cycle. First, as indicated by the reference number 102, the IC to be designed is specified by a specification document.
Then, the IC design is reduced to an HDL code, as indicated by the reference number 104. This level of design abstraction is referred to as the Registered Transfer Level (RTL), and is typically implemented using a HDL language such as Verilog-HDL (xe2x80x9cVerilogxe2x80x9d) or VHDL. At the RTL level of abstraction, the IC design is specified by describing the operations that are performed on data as it flows between circuit inputs, outputs, and clocked registers. The RTL level description is referred to as the RTL code, which is generally written in Verilog or in VHDL.
The IC design, as expressed by the RTL code, is then synthesized to generate a gate-level description, or a netlist. This is referred to by the reference number 106 of FIG. 1. Synthesis is the step taken to translate the architectural and functional descriptions of the design, represented by RTL code, to a lower level of representation of the design such as a logic-level and gate-level descriptions.
The IC design specification and the RTL code are technology independent. That is, the specification and the RTL code do not specify the exact gates or logic devices to be used to implement the design. However, the gate-level description of the IC design is technology dependent. This is because, during the synthesis process, the synthesis tool uses a given technology library, 108 of FIG. 1, to map the technology independent RTL code into technology dependent gate-level netlists.
An integrated circuit chip (hereafter referred to as an xe2x80x9cICxe2x80x9d or a xe2x80x9cchipxe2x80x9d) comprises cells and connections between the cells formed on a surface of a semiconductor substrate. The IC may include a large number of cells and require complex connections between the cells.
A cell is a group of one or more circuit elements such as transistors, capacitors, and other basic circuit elements grouped to perform a function. Each of the cells of an IC may have one or more pins, each of which, in turn, may be connected to one or more other pins of the IC by wires. The wires connecting the pins of the IC are also formed on the surface of the chip.
A net is a set of two or more pins which must be connected, thus connecting the logic circuits having the pins. Because a typical chip has thousands, tens of thousands, or hundreds of thousands of pins, that must be connected in various combinations, the chip also includes definitions of thousands, tens of thousands, or hundreds of thousands of nets, or sets of pins. The number of the nets for a chip is typically in the same order as the order of the number of cells on that chip. Commonly, a majority of the nets include only two pins to be connected; however, many nets comprise three or more pins. Some nets may include hundreds of pins or thousands or tens of thousands to be connected. A netlist is a list of nets including names of connected pins or a list of cells including names of nets that connect to pins of cells.
A netlist may be generic or technology specific. A generic netlist is a netlist created from the RTL code that has not yet been correlated with a technology specific library of cells. A technology specific netlist, or a mapped netlist, is a netlist created after the IC design has been mapped to a particular technology-specific library of cells. Continuing to refer to FIG. 1, after the synthesis of the design, the gate-level netlist is verified 110, the layout of the circuits is determined 112, and the IC is fabricated 114.
At the RTL level, designers must take all key design decisions such as design hierarchy and partitioning, clocking scheme, reset scheme, and locations of registers.
All those decisions are contained and reflected in the RTL code. The RTL code is technology independent, as well as independent from design tools.
As a result, some characteristics of the RTL code can strongly influence further design steps, including logic synthesis, gate-level simulation, static timing analysis, test insertion and layout. Unexpected problems and difficulties with the IC design can be encountered at any of these steps and cause implementation obstacles impacting project schedules and costs.
Some problems, referred to as showstoppers, may render the design not feasible for fabrication. For example, it may be realized during clock distribution that the design uses an unsupported clocking scheme, such as clock signals that are gated xe2x80x9con the flyxe2x80x9d whenever needed. A clock signal is gated xe2x80x9con the flyxe2x80x9d when a gate, usually an AND gate, is used to turn on a clock signal only when need for a particular sub-circuit but turned off the rest of the time. This is a common technique to reduce power consumption. The problem arises if and when the same clock signal is needed elsewhere. Then, clock distribution cannot be made, and the RTL code needs significant re-work.
Other design problems may present implementation obstacles requiring the engineering efforts to be much higher than expected. For example, it may be realized during logic synthesis that the design is partitioned in a very xe2x80x9csynthesis unfriendlyxe2x80x9d manner. In such a case, the automatic features of the synthesis tools cannot be used, and, in its place, a lot of manual work is required to meet timing and other parameters.
Encountered late in the design cycle, such problems can greatly impact project schedules and design cost. The later the problems are discovered, the more significant the impact and the higher the cost in time and expenditure to correct the error. For example, timing or routability problems encountered during layout can require a new run through logic synthesis, gate-level verification, and test logic insertion. Modifying the RTL code late in the design process is generally the worst case scenario because once the RTL code is modified, all design steps must be re-run, including the RTL functional validation. For many design projects, RTL modification is not even a viable option.
To identify the potential problems with the IC design as early as possible, RTL code can be analyzed, manually or automatically. However, some design issues can be missed if the RTL code itself is used to analyze the design. In addition, some constructs of the languages used for the RTL code, such as Verilog and VHDL, leave room for more than one interpretation when implementing them in hardware. These shortcomings exist because the languages used for the RTL code, Verilog and VHDL, lack formerly-defined synthesis semantics. These languages lack the formerly-defined synthesis semantics because they were developed as simulation languages before logic synthesis tools were available.
The general purpose of the present invention is to provide the means to analyze IC designs early in the design process in order to allow correction of problems early on. Therefore, an object of the present invention is to extract critical design information from RTL code and identify early in the design process issues that can impact further design steps. The size and complexity of RTL code make xe2x80x9cmanualxe2x80x9d RTL analysis unworkable.
Based on the context described above, another object of the invention is to define tools referred to as xe2x80x9csynthesis script generation toolsxe2x80x9d, that automatically extract from RTL code design information that is required for synthesis, including design hierarchy, clock sources and fanouts, hierarchy purity of modules, and types of pins that drive module outputs, and create optimized scripts to synthesize the design in a given target technology, using a given target synthesis tool. Purity of hierarchy refers to whether a particular level includes sub-designs only, logic only (if leaf), or sub-designs mixed with logic. Types of pins that drive module outputs may be registered or unregistered, and driven or not-driven by a flipflop.
It is a further object of this invention to provide a method of accessing the generic netlist from the Synopsys Design Compiler or similar synthesis tools. As discussed above, a generic netlist is a netlist created from the RTL code which has not been correlated with a technology-specific library. For example, RTL code describing a select function between sixteen input signals to a single output signal may be implemented as a multiplexer circuit (a xe2x80x9cMUXxe2x80x9d). A generic netlist may represent the sample circuit as a 16xc3x971 MUX having a 16 input signals, four input selection signals, and one output signal. In contrast, a technology-specific netlist may represent the sample circuit as a cascade of several 4xc3x971 MUX""s.
Another object of the present invention is to extract critical design information from a generic netlist and identify as soon as possible issues that can impact further design steps. Analysis of RTL code may miss some design issues. These potentially problematic issues which can be missed at the RTL code analysis phase can be identified if the IC design is analyzed at the generic netlist level.
Accordingly, it is a further object of this invention to provide a method of accessing the generic netlist from the Synopsys Design Compiler or similar synthesis tools. Before information can be extracted from a generic netlist, one must first have access to the generic netlist.
Another object of the present invention is to effectively analyze mapped designs for buffering trees and determine their structure, their root pins, or active transitions or levels on their leaf pins.
Another object of the present invention is to utilize mapping techniques to maintain the known names of the source pins of the clocks even after the initial mapping process. During initial mapping process, the names of cells and pins are assigned by synthesis tools. Because of the name assignments, the names of the source pins of internal clocks are modified and are no longer available for resynthesis and characterization steps.
Another object of the present invention is to increase the speed in which large designs are synthesized by creative use of dc_shell command to characterize the modules of the design. As discussed herein, synthesis of IC designs involve iterations of the following two steps: bottom-up synthesis of sub-modules, and top-down characterization. The top-down characterization step can be improved by characterizing a list of module instances rather than characterizing one module at a time.
Another object of the present invention is to define a practical technique to synthesize the IC design having DesignWare modules. As discussed herein, DesignWare modules are typically predefined structured logic circuits with predefined characteristics. Because they are predefined to be general logic elements, DesignWare components may include circuits and pins which may be not necessary such as unused I/O ports. The present invention discloses techniques, including ungrouping and resynthesis to improve the performance of the synthesis script.
These and other aspects, features, and advantages of the present invention will be apparent to those persons having ordinary skill in the art to which the present invention relates from the foregoing description and the accompanying drawings.
Accordingly, the present invention is a method of efficiently characterizing modules of an integrated circuit (IC) design using a logic synthesis tool comprising the steps of defining a list of instances of the modules to characterize, and characterizing entire modules of said list of instances of the modules using a single invocation of characterize command of the logic synthesis tool.